This is also what I learned the hard way. In many situations the customer doesn’t say what they really want. There are a lot of reasons why. I usually have to write down a lot of hypotheticals. If X is the primary concern, we should do Y. If U is the issue, we should do V.
I wouldn't expect customers to say what they really want. They are looking for faster horses after all. But law professors? Among all professors, law professors should be the ones saying what they really want.